In U.S. Pat. No. 3,981,481 granted Sept. 21, 1976, filed Oct. 8, 1974, a railway tank car bottom outlet valve assembly is disclosed in which the outlet chamber and a portion of a bottom operator for the valve closure drop off upon impact to the outlet chamber. In FIGS. 5-18 of this patent, movement of the valve closure between open and closed positions requires that a depending portion of the valve closure engage a retainer or spider to prevent rotation of the valve closure and allow the valve closure to move between open and closed positions.
While the construction shown in this patent represented a significant advance in the art in providing an outlet chamber which would shear off upon impact and prevent the lading from escaping during such an impact to the outlet chamber, the relatively thick mounting flange required in this construction necessary to mount both the valve seat and the spider or retainer to prevent rotation of the valve closure, necessitated a mounting flange which extends more than one (1) inch below the bottom surface of the tank bottom.
AAR and DOT regulations concerning projections extending below the tank bottom prohibit the mounting flange from extending more than one (1) inch below the tank bottom without additional shear-off protection.
Another problem with the construction illustrated in the '481 patent is that the projections extending downwardly from the valve closure to prevent rotation of the valve closure also function to stop vertical movement of the valve closure in the full open position. In order to have satisfactory unloading rates, the amount of travel of the valve closure and the depending extensions must be of the order of 11/2-2 inches. If these valve closure extensions were to be reduced in vertical extent sufficient to have a mounting flange extend no more than one (1) inch below the tank bottom, the amount of available travel of the extensions would be less than one (1) inch. This would result in too low lading unloading rates and loading rates for commercial acceptance.
It is unpleasant and to some extent a safety hazard for workmen to be located within the tank during installation.